Many designs of scanning equipment make use of a linear CCD sensor which is basically a 1.times.n array of photocells coupled to an n cell shift register. For some applications such as film scanners, n may be several thousand photocells. For color reproduction, three independent 1.times.n arrays may be formed on a single die. Light from the illuminated subject is focused onto the line of photocells, which generate and accumulate charges in response thereto. The accumulated charges are then transferred into the respective cells of the shift register and read out one cell of pixel information at a time. If the object to be scanned is two dimensional, such as a photographic negative, there must be relative motion between the CCD sensor and the object, and the cycle of photo exposure, transferring and reading out repeats continually until the object is scanned. With a 4000 cell linear sensor providing 4000 pixel resolution across a 35 mm motion picture frame, comparable vertical resolution requires about 3000 lines, hence 3000 cycles. Rather than vary the scanning rate for exposure time control, it is more convenient to build into the CCD sensor a type of exposure gate, called a lateral overflow gate. When the gate is open, the charges generated by the photocells are continually drained off. When the gate electrode voltage is at a first level +V of about +3 to +8 volts, a "high voltage", all charges generated by the photocells are continually drained off. With the high voltage applied to the gate, the gate is defined as being fully open. As the gate electrode voltage is lowered, charges can accumulate to a maximum determined by the gate electrode voltage. Typically the voltage is lowered to a second level of -V, "a low voltage", of about 0 to -4 volts. At this second voltage level the gate is defined as being fully closed. The second level voltage -V determines the maximum allowed charge accumulation and may varied to control blooming in the image produced at the CCD sensor output.
In the interest of higher speed, it is usual to overlap the two time consuming parts of the cycle; while one line is being read out of the register, the next line is being generated by the photocells. As machines have gotten faster, a problem has arisen with respect to the lateral overflow gate. The gate electrode voltage normally rises to the fully open value during the period between lines when there is no output. When the gate electrode voltage is lowered to -V, the shift register is normally in the readout part of the cycle. Transients created by closing the gate tend to create an anomaly or "artifact" in the output. If the transient appears repeatedly at the same part of the cycle, streaks can appear on an output print.
There is a strong need, therefore, to eliminate the visible effects of closing the lateral overflow gate during register readout.